1. Field of the Invention
The present invention relates to an inductance device suitable for electronic instruments required to be made smaller in particular, such as mobile phones, digital cameras, mobile instruments, and notebook PCs.
2. Description of the Prior Art
Known as this kind of inductance device is one using a drum core made of ferrite, in which a ring core made of ferrite concentrically covers the outer periphery of a magnetic gap existing between its upper flange and lower flange, so as to prevent magnetic fluxes from leaking from the gap, and increase permeability.
It is necessary for thus configured inductance device to have at least a predetermined clearance between each flange of the drum core and the ring core. This is because of the fact that both of the drum core and ring core formed from ferrite have a high permeability, so that magnetic saturation will occur if the clearance therebetween is too small, whereby a predetermined inductance value may not be obtained.
Since the ring core incurs a dimensional tolerance during the making thereof, it is quite difficult for the drum core and ring core to be positioned accurately when concentrically attaching and securing the ring core to the outer periphery of the drum core. As a result, the above-mentioned clearance may vary among devices, whereby electric characteristics may differ from device to device.
Known as a technique which can overcome the problem of inductance devices mentioned above is a high-frequency transformer disclosed in Japanese Patent No. 2868064 (hereinafter referred to as “reference 1”).
The high-frequency transformer disclosed in reference 1 is configured such that a drum core and a terminal board, and the terminal board and a holder are positioned with respect to each other by their respective predetermined mating forms, whereas a ring core is inserted into a through hole of the holder while in thus positioned state. As a consequence, the relative positional accuracy between the drum cores and ring cores can be improved, whereby the above-mentioned problem of varying clearances and electric characteristics among the devices can be overcome.
However, since the flange (upper flange) of the drum core farther from the terminal board mounting the drum core is bonded to the upper end of the ring core by an adhesive, while an assembling operation is carried out using a holder for holding the drum core and ring core, the high-frequency transformer disclosed in reference 1 may be problematic in that the number of parts increases while the manufacturing process is complicated.
Therefore, as disclosed in Japanese Utility Model Publication No. HEI 3-46491 (hereinafter referred to as “reference 2”), it has been known to use a tape-like magnetic member instead of the ring core, and wind it about the drum core while extending it between the upper flange and lower flange of the drum core, thereby covering the outer peripheral side of the magnetic gap in the drum core.
On the other hand, as disclosed in Japanese Utility Model Publication No. SHO 64-2420 (hereinafter referred to as “reference 3”), it has been known to mount a hard cover made of a synthetic resin mixed with ferrite powder onto a magnetic core wound with a coil by using the spring elasticity of the cover.
Since the technique disclosed in reference 2 requires an operation of winding a tape-like magnetic member about the drum core while extending it between the upper flange and lower flange thereof, the assembling operation is not easy in a minute inductance device whose upper flange and lower flange have a gap of about several millimeters or less therebetween in particular.
The technique disclosed in reference 3 shields most part of the outer face of the magnetic core with a cover containing magnetic powder mixed therein, whereby the total size of the device may become large when applied to a magnetic core having upper flange and lower flange in particular.
Further, the techniques disclosed in references 1 to 3 are susceptible to mechanical shocks such as falling and punching. Namely, whether drum cores or ring cores, magnetic cores used in inductance devices in general are formed by baking ferrite or the like and thus are susceptible to mechanical shocks such as falling and punching and are likely to be damaged though exhibiting a hardness to some extent. The tape-like magnetic member wound about the magnetic core in reference 2 and the hard cover with spring elasticity shielding most part of the magnetic core in reference 3 may not always improve the resistance to shocks.